In general, inductive sensors for finding a metallic, electrically conductive object in a surrounding medium are known, for example, from U.S. Pat. No. 5,729,143.
Sensors which detect the presence or movement of a non-metallic object through a flat or planar article, for example, through a panel that is not transparent to optical radiation are usually constructed on the principle of capacitance measurement. For example, capacitive touch pads are a known application as known e.g. from DE 103 24 579 A1, or capacitive proximity sensors as described, for example, in DE 101 31 243 C1 in which a sensor with sensor electrodes and an associated control circuit for evaluating the sensor signals are provided, wherein a further electrode surrounds the sensor electrode. In these applications, the sensor unit is firmly connected to the panel to be penetrated and, as viewed from the sensor unit, the object to be detected moves behind this panel. Due to the mechanical arrangement, the sensor and the panel have a fixed capacitance relative to one another which is reflected in the measurement value as a constant basic capacitance.
In a further application there are sensors which must be moved over the planar article or the panel in order to locate objects lying therebehind, as is known e.g. from EP 0 657 032 B1 and EP 1 740 981 B1. This includes so-called stud-detectors. Stud-detectors are general ancillary devices for DIY practitioners and professionals, for example, for detecting studs, posts, pipes or power lines behind a closed wood cladding or a covering in ready-built houses. For this purpose, the sensor is guided over the wall. It measures with an electrode the capacitance relative to the wall. If a wooden post, a pipe or a power line is situated in the detection range, due to the change in the dielectric material, the capacitance increases. This is suitably evaluated and displayed to the user. Provided the sensor is moved at exactly the same spacing from the flat article or panel, the capacitance between the sensor and the flat article or panel does not change. It enters the measurement signal only as a constant value, as in the first two examples. However, with the example of the stud-detector, it can be understood that the maintenance of a constant spacing is almost impossible in practice and therefore the basic capacitance resulting from the wall construction changes significantly as a function of the spacing. Therefore, in the following description of the invention, the stud-detector is also selected as an exemplary embodiment.
From DE 20 2004 011 921 U1, a coil arrangement is known, in particular for a metal detector, consisting of a transmitting coil and a receiving coil arranged orthogonally to the transmitting coil. The distinctive feature of this orthogonal arrangement lies therein that the two coils can be positioned relative to one another so that without external influences on the receiving coil, no voltage induced by the transmitting coil operated with an electronic signal can be measured. A metallic, electrically conductive object in the detection range of the sensor bends the field lines of the emitted electromagnetic field permeating the receiving coil, so that an induced voltage can be measured on the receiving coil. This voltage is referred to below as the measurement signal.
If, for example, a steel reinforcement in concrete would have been detected with such an inductive sensor, the sensor is guided along the concrete surface. A maximum measurement signal occurs when the reinforcing steel has a minimum spacing from the sensor arrangement. Accordingly, the presence of a reinforcing steel is interpreted at the local positions where maxima of the measurement signal occur.
If, for example, a steel reinforcing grid cast in concrete is measured in this way with an inductive sensor, then eddy currents which occur in a metallic, electrically conductive object due to the emitted electromagnetic field of the inductive sensor can impair the measurement such that, for example, a minimum of the measurement signal occurs over a steel reinforcing segment of the grid and a maximum of the measurement signal occurs over the hollow sites in the middle of the grid. Circulating currents along the grid structure which act like electrical loops are responsible for this effect.
False interpretations can thus arise since the spatial position of a reinforcing steel segment is interpreted at the site where a maximum measurement signal occurs. The unwanted occurrence of measurement signal maxima or measurement signal minima at unfavourable sites of a grid-like metallic, electrically conductive object which can lead to false interpretations is referred to below as the grid effect.
DE 37 33 529 A1 discloses an inductive metal detecting device with at least four exciter coils and at least one receiving coil. The four exciter coils the centre points of which are arranged on a closed circular path are controlled in a periodic sequence with different currents such that a resultant magnetic field largely continuously changes its direction at an amplitude that remains substantially constant at a repetition frequency corresponding to said sequence. The exciter coils are arranged either in a plane spanned by the circular path or are each orthogonal to the plane.
US 2009/0219027 A1 discloses a multisensor for detecting and identifying unexploded ordnance. The multisensor has at least two transmitting coils arranged orthogonally to one another and a plurality of receiving coils arranged orthogonally thereto. The receiving coils are arranged in two planes spaced from one another and extending orthogonally to the transmitting coils, wherein the transmitting coils are arranged between the receiving coils.
From DE 10 2010 007 620 A1, there is known a proximity sensor with a first transmitting coil, a second transmitting coil, at least one receiving coil, an exciter device which is connected to the first and second transmitting coil, and an evaluation device which is connected to at least one transmitting coil and/or the exciter device and to the at least one receiving coil.
EP 1 092 988 A1 discloses an inductive sensor arrangement for detecting metallic objects hidden by a surrounding medium. The sensor arrangement has two transmitting coils arranged adjacently to one another at a defined spacing and in one plane, and a pair of receiving coils, each receiving coil being arranged in a respective field of the transmitting coils such that, in a setting that is free from iron-containing objects, no voltage is induced in them.
DE 10 2009 010 943 A1 relates to an operating method and a coil arrangement for a magnetic sensor for detecting metallic objects in a subsurface medium with a first main coil, at least two coils of a first coil group arranged in an interior space of an expanded winding cylinder of the first main coil and at least two coils of a second coil group arranged in the interior space of the expanded winding cylinder of the first main coil, and having a different orientation from the coils of the first coil group.